Atomic structure of the fluctuating (√3x√3)R30 phase of the Sn/Ge(111) system.

H. ASCOLANI; J. AVILA; G. LE LAY; M.C. ASENSIO

San Carlos de Bariloche, Argentina

Conferencia; 13th International conference on solid films and surfaces; 2006

Centro Atomico Bariloche CNEA

In this work we have determined the atomic structure of the (√3x√3)R30 phase (√3 from hereafter) of the 1/3 monolayer Sn/Ge(111) surface  by  using chemical-shift resolved photoelectron diffraction. The nature of this phase has been highly controversial since it was found that it undergoes a reversible transition to a (3x3) phase upon cooling below 220 K.  STM images measured at room temperature (RT) show all equivalent Sn adatoms forming √3 reconstruction with one Sn atom per unit cell, while, at low temperatures (LT), they show three inequivalent Sn atoms in the (3x3) unit cell. On the base of STM results a surface-charge-density-wave model has been proposed. However, core-level photoemission results have strongly challenged the idea of a displacive transition with a RT phase being a typical √3 reconstruction. The fact that the Sn-4d photoemission peak has two components with an intensity ratio 1:2, regardless of the sample temperature and surface reconstruction makes irreconcilable the reported phase transition with a displacive mechanism.  The experimental and theoretical results collected up to now indicate that the dynamical-fluctuations model is the one, which correctly describes the atomistic mechanism of the transition. In this model the stable surface is considered to be a (3x3) one, along the whole temperature range. At LT the system is frozen in the stable reconstruction, while, above the transition temperature, it starts oscillating with increasing frequency, between the two well differentiate vertical positions occupied at LT. Regarding the atomic structure of the LT phase, several diffraction experiments have shown that the (3x3) reconstruction is corrugated with the Sn adatoms distributed between two inequivalent vertical positions.  The most  popular model for the (3x3) reconstruction is  the 1U2D, where one Sn atom is displaced upwards and the other two displaced downwards. However, very recently, several experiments have reported that the 2U1D  model is the correct one, coinciding with what we have initially suggested, based on early X-Ray Surface Diffraction and Photodiffraction results.                  In this work we address the issue of the atomic structure of the fluctuating √3 and  the origin of the two components of the two components of the Sn-4d  photoemission peak. We present results which clearly favour the 2U1D model and contradicts the relationship between components and inequivalent adsorption sites proposed in previous studies. TIPO DE PRESENTACION:  POSTER